Basic Genetics For The Family Historian

Elementary genetics provides us with the fact that the 23rd
chromosome, in the human genome, is the chromosome that determines sex.Males have both an "X" and a "Y" 23rd chromosome, but
females only carry an "X" for their 23rd chromosome.

The human egg will becomes a female embryo if the male
sperm that initially reaches the egg carries an X-chromosome.The egg will becomes a male embryo if the male sperm that initially
reaches the egg carries a Y-chromosome.Thus
you can see the Y-chromosome is passed down from generation to generation only
through the male line.In
order to better understand how we arrived at this point, we need to reach for
the next level.

The complete set of instructions for making an
organism is called its genome.Found
in every nucleus of a personís many cells, the human genome consists of
tightly coiled threads of deoxyribonucleic acid (DNA) and associated protein
molecules, organized into structures called chromosomes.

In humans, as in other higher organisms, a DNA
molecule consists of two strands that wrap around each other to resemble a
twisted ladder whose sides, made of sugar and phosphate molecules, are connected
by rungs of nitrogen, containing chemicals called bases.Each strand is a linear arrangement of repeating
similar units called nucleotides, which are each composed of one sugar, one
phosphate, and a nitrogenous base.Four
different bases are present in DNA: adenine (A), thymine (T), cytosine (C), and
guanine (G).The particular
order of the bases arranged along the sugar- phosphate backbone is called the
DNA sequence. These sequences specify the exact genetic instructions required to
create a particular organism with its own unique traits.

The two DNA strands are held together by weak bonds between
the bases on each strand, forming base pairs (bp).Genome size is usually stated as the total number of
base pairs.The human genome
contains roughly 3 billion base pairs (bp).

These three billion base pairs (bp) in the human genome are
organized into 24 distinct, physically separate microscopic units called
chromosomes.All genes are
arranged linearly along the chromosomes.The nucleus of most human cells contains 2 sets of chromosomes.One set is provided by each parent.Each set has 23 single chromosomes; 22 autosomes and an X or Y sex
chromosome.A normal female
will have a pair of X chromosomes in this 23rd chromosome; a normal
male will have an X and Y pair.